McShane tells Pop Goes the News that his character will only appear in one episode of the series, but he'll be bringing someone else with him.
"I am responsible for bringing somebody back that you think you're never going to see again. I'll leave it at that.
"[It's] a one-off episode," he said. "That's why I did it. But I can't reveal anything, otherwise I will never work again."
Potential SPOILERS follow.
The idea of McShane bringing someone back will immediately have fans jumping to Jon Snow, but that seems unlikely. The series already positioned Melisandre to resurrect Jon, and that's likely a plotline that will take more than an episode to resolve,
The most plausible theory so far is that McShane will be playing Elder Brother, Septon Meribald, or a composite character based on both. In the novels, Meribald is a wandering Septon in the Riverlands who brings Brienne, Podrick Payne, and Ser Hyle Hunt to the Quiet Isle, where they meet Elder Brother. Elder Brother is said to have healing powers, and there's a gravedigger on the Isle who – while not named – fits the description of Sandor Clegane.
Rory McCann, the actor who plays the Hound, is rumored to have been spotted on the set of Game of Thrones Season 6 during filming. There's also been a lot of talk coming from the set of storylines returning to the Riverlands, so a visit to the Quiet Isle seems quite likely. Could the Clegane Bowl fans have been clamoring for follow?
Game of Thrones returns to HBO in 2016.